The present invention relates to a thin film transistor and a method of manufacturing the same and, more particularly, to a polysilicon thin film transistor formed on an insulating substrate such as a glass substrate, and a method of manufacturing the same.
In a thin film transistor having a polysilicon channel region, the field-effect mobilities of electrons and holes are larger than in a thin film transistor having an amorphous silicon channel region. The polysilicon thin film transistor therefore has a high current drive capability and is being used in liquid crystal display apparatuses and the like.
Regardless of its excellent characteristics, however, the polysilicon thin film transistor suffers the following problem.
More specifically, in the polysilicon thin film transistor, its characteristics and particularly its threshold voltage are greatly influenced by defects, impurities, and the like in the channel region. As a result, the threshold voltage may greatly shift from a value necessary for the operation of the transistor circuit. For example, in an n-channel transistor, the threshold voltage shifting to the negative side with respect to 0V makes the transistor have normally ON characteristics, failing normal switching.
To solve this problem, the threshold voltage is controlled by implanting, e.g., boron ions in the channel region using an ion implanter or an ion doping apparatus. According to this method, however, ion implantation or ion doping must be performed in addition to formation of the channel region. Accordingly, the number of steps of manufacturing a liquid crystal display device or the like increases, and the throughput as the productivity per unit time decreases. In addition, this method requires another apparatus, and thus the manufacturing cost increases.
The amount of boron ions implanted in the channel region to control the threshold voltage is very small. However, when an ion implanter or ion doping apparatus is used, the boron dose cannot be accurately controlled, and the thin film transistor cannot be manufactured with high reproducibility and high stability.
For example, boron ions are implanted in the channel region using an ion implanter or an ion doping apparatus in manufacturing a thin film transistor having a structure in which a silicon nitride film, a silicon oxide film, and a polysilicon thin film forming the channel region are sequentially stacked on a glass substrate. In this case, the boron concentration distribution shown in FIG. 1 is generally obtained.
FIG. 1 is a graph showing the boron concentration distribution obtained when a thin film transistor is formed by a conventional method. In FIG. 1, the abscissa represents the boron dose, and the ordinate represents the distance from the glass substrate.
As is often the case with the conventional method, the boron concentration in the silicon oxide film gradually decreases from the polysilicon thin film toward the silicon nitride film as shown in FIG. 1. It is noted that a high boron concentration is obtained in not only the polysilicon thin film but also the interface region between the silicon oxide film and the silicon nitride film. This is because the surface of the silicon nitride thin film is contaminated before forming the silicon oxide film.
The threshold voltage does not greatly vary between thin film transistors as far as the amount of boron ions implanted in the silicon oxide thin film and the silicon nitride thin film is very small or constant.
However, when an ion implanter or ion doping apparatus is used, it is difficult to prevent a non-negligible amount of boron ions from being implanted in the silicon oxide thin film and the silicon nitride thin film. The amount of boron ions implanted in the silicon oxide film and the silicon nitride film greatly depends on implantation conditions such as the boron ion acceleration voltage, and these implantation conditions cannot be kept unchanged. For this reason, in the conventional method, the threshold voltage greatly varies between thin film transistors.